I don't know what would cause the nitrate to show 0, especially if your tap water has nitrates. If it was 10 or so before taking out 20% of water, it should have still been very near 10 after adding water back (technically, you have about 4 parts - 80% of tank water left - at 10 and 1 part - the 20% tap water you added back - at 5 which would mean the new nitrate reading should be 9). The dechlorinator really should not have any effect on any of the readings.
I may not have been clear on the Nitrate being around 20ppm before I actually did the water change, tested immediately after (mere minute or so) and obtained 0ppm nitrate. Was surprised by the result (as mentioned I expected 10ppm) as my tap water has 5ppm - of itself I retested tapwater just to be sure. I've read some threads on the aquaria newsgroups before where people have stated that taking readings immediately after a water change (small or large) will have erratic results. So I decided to retest the water some 5 hours later and the levels were more of what I would have expected (10ppm Nitrate) or near enough the 9ppm you estimate.
AARONC: Sorry, I don't know if the sponge in my Eheim Classic external cannister is a 'nitrate removal' sponge or not. It has several sponges, namely: coarse, fine and charcoal. Also has two other kinds of filter media (ceramic rings, and these pebble like things that are 'sintered quartz glass' based - whatever that means?!?). The sponges I think are just different levels of mechanical filtration and of course a possible bacteria colonization area.
Readings this evening: Ammonia = ~4.0, Nitrite = 0, Nitrate = ~10, pH = 7.8.
I'll have to give it another day or so to see where ammonia and nitrates are at. This 47G tank in all has had some 40ml of 4% ammonia put into it, it's going somewhere. And if Nitrates move back up again to 20ppm, then categorically the "mature" filter sponge I dropped into my cannister from my friends tank - has likely been the culprit processing the ammonia fully.
ENVIRONMENT:
TANK: Perfecto 47G "Column" (30" H x 20" W x 18" D).
FILTER: Eheim Classic External Cannister #2217 (ceramic rings, coarse sponges, sintered glass pebbles, fine sponge, charcoal sponge)
PUMP: Rena 300 Air on a single 14" bubble wall.
HEATER: Hydor External In-Line
PLANTS: None presently, planning on having some
SUBSTRATE: gravel (black, multicolour accents)
ORNAMENTS: rock, pebbles, plastic shipwreck and divers helmet
Please ponder the following readings:
Date Ammonia Nitrates
13/01/2007 5.0 <<< Boosted ammonia prior day
14/01/2007 5.0 <<< Added "mature" filter from aquaclear into my Eheim cannister
15/01/2007 5.0
16/01/2007 4.0
17/01/2007 3.5
18/01/2007 2.5
19/01/2007 2.0 10
20/01/2007 2.0 20 <<< Boosted ammonia in evening to 8.0ppm (or higher)
21/01/2007 8.0 0 <<< Still had High reading in morning
21/01/2007 4.0 10 <<< Did 20% water change at noon (to get back into readable range on my test card), obtained these readings at dinnertime
22/01/2007 3.5 10
23/01/2007 3.0 10
24/01/2007 2.5 20
25/01/2007 2.0 20
NOTES for my "fish-less" cycle:
* Nitrites reading has always been at 0 ppm.
* Tap water has 5.0ppm Nitrates, 0ppm Ammonia
* Test kit is API Freshwater Master Test Kit
* Tank was started on 27/12/2006. Fishless cycle 'stalled' so I retried 12/01/2007 per rdd's advice (not blaming, rdd's helping me!!!)
* Water conditioner used is "Big Al's Water Conditioner"
QUESTIONS (and concerns):
1) Ammonia reduction is occuring but somewhat slowly compared to that indicated in other posts and from my understanding of rdd's original post. Nitrates appear to be steadily building as well. I believe this to be as a result of having a "mature" filter media being added into my "new/virgin" tank setup. i.e. was processing ammonia fully in its original tank, now picking up where it left off only now in my tank. Can I consider my tank cycled?
2) If above answer is "Yes", Why is it taking sooooo long for ammonia to drop (impression is it should do so within 8 to 12 hours, 24 hours at most)?
3) Can I now add fish? I know exactly what fish-load the "mature" filter had in its original tank... if that helps the equation any.
4) If yes to #3 above, do I dose "Ammo-lock" to render the present ammonia "safe" and begin stocking this weekend?
5) How many fish can I safely or reasonably add in a single go?
6) When would it be considered safe to add any more beyond that?
** The mature filter addition after my stalled fishless cycle has me thinking I'm out of fishless cycling territory... thus all the questions as to how to proceed safely. **
CONSIDERING STOCKING:
6 x Dwarf Neon Rainbo, 1 x Red Tail Black Shark, 4 x Dwarf Gourami, 3 x Otto, 6 x Flame Tetra (orange von Rio)
<also pondering one of following>
1 x Bala Shark, OR 3 x German Blue Ram, OR 2 x Krib
The kids want fish, the wife wants fish (she bought this big prezzie for me), heck... I want fish too!!! Advice or responses to my concerns above are very much appreciated.